Polymers are generally characterized by their bulk properties such as tensile strength, yield stress, hardness, stiffness, elongation and gas permeability. Manufacturers use these properties to determine whether a particular polymer might be useful in a particular application. If, for example, a material that is hard and impact resistant is required, for use in say motorcycle helmets, a polymer that exhibits those bulk properties will be selected. If the intended use requires flexibility, toughness and elongation, as might be case with expandable coronary stents, a different type of polymer will be chosen.
Polymer characteristics are affected by the monomer types and composition, the polymer architecture and the molecular weight. The crystallinity of the polymer, an important factor in polymer biodegradation, varies with the stereoregularity of the polymer. For example, racemic D,L poly(lactide) or poly(glycolide) is less crystalline than the D or L homopolymers. Poly(lactide) (PLA) and its copolymers having less than 50% glycolic acid content are soluble in common solvents such as chlorinated hydrocarbons, tetrahydrofuran and ethyl acetate while poly(glycolide) (PGA) is insoluble in common solvents but is soluble in hexafluoroisopropanol.
The bulk properties of polymers can, however, change with time, a process known as aging. Aging can render a polymer unsuitable for its originally intended purpose and possibly cause a construct comprising that polymer to fail in use.
Polymers age by physical, chemical and/or electrical processes. Chemical aging results from exposure of a polymer to external factors such as air (oxygen), moisture, solvents, radiation, heat and light. Electrical aging results from voltage-induced stress that occurs at voltages usually in excess of about 3 kilovolts. Physical aging, which is the primary focus of this invention, results from residual and applied stresses.
This is true with polymer-coated implantable medical devices, e.g., drug-eluting stents (DESs), as well. The efficacy of DESs is related to their ability to release drugs in a controlled manner. One way this is accomplished is to include on the DES a rate-controlling layer, e.g., a topcoat layer, that is disposed over a drug reservoir layer and which comprises one or more polymers selected for their ability to mediate release of a particular drug or drugs from the underlying reservoir layer.
Another way to control drug release from a stent is by putting drugs in a drug reservoir layer that includes a polymeric matrix that mediates the release rate of the drug. Indeed, by manipulating the drug-to-polymer ratio, drug release can be controlled.
In both of these situations, however, the choice of polymer greatly affects the release of drugs from the device as well as the long term stability of the polymer on the device.
What is needed, therefore, is a method of mitigating the aging process of polymers so as to extend the useful life of coated medical devices as well as add an additional level of control over release rates of drug eluting devices. The present invention provides coatings that solve these and other problems in the art.